<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
    <style></style>
<!--     内部式：-->
    <style type="text/css">
        body{
            /*background-color: grey;*/
            background-image: url("1.png");
            background-repeat: no-repeat;
        }
        #div_id1{
            background-color: red;
            font-size: 72px;
            font-weight: lighter;
        }
        .div_class1{
            background-color: green;
            border:solid 1px yellow;
        }
    </style>
<!--外部式-->
    <link rel="stylesheet" type="text/css" href="main.css">
</head>
<body>
<!--内嵌式-->
<div style="background-color: blue;">div</div>
<!--#通过id找到这个div,具有唯一性-->
<div id="div_id1">div2</div>
<!--#类选择器统一管理div3,div4-->
<div class="div_class1">div3</div>
<div class="div_class1">div4</div>
</body>
</html>